It would seem to make more sense, then, to call the 9th chord a G7+9, for example, wouldn't it? The fact that there is a discussion on it means that reasonably experienced guitarists are not fully sure of the difference between a 2nd and a 9th. Inserting '7' into the description should help to clarify the difference.

I really dislike the "+" sign to mean add to, as the "+" has another convention to mean "augmented." Some folks also use it to mean raising anything (which I also dislike) so C7+9 equals C7#9...very confusing...I leave + for augmented chords and that's it.

I think the current convention is already pretty easy to understand...the # assumes all voices below it are fair game...so a C11 can have the R, 3, 5, b7, and 9th as well...Letter + number seven or greater = dominant chord. If it says "maj" then it's a maj7 included, not a b7.

I really dislike the "+" sign to mean add to, as the "+" has another convention to mean "augmented." Some folks also use it to mean raising anything (which I also dislike) so C7+9 equals C7#9...very confusing...I leave + for augmented chords and that's it.

I think the current convention is already pretty easy to understand...the # assumes all voices below it are fair game...so a C11 can have the R, 3, 5, b7, and 9th as well...Letter + number seven or greater = dominant chord. If it says "maj" then it's a maj7 included, not a b7.

so Cmaj11= C E G B D F

C11= C E G Bb D F

...bearing in mind that in practice, the 3rds would be omitted from those chords . The E-F is a minor 9th, a "nasty" sound - the classic "avoid note" (which is really an "avoid interval").
And in Cmaj11, too, the B-F is the tritone, the central part of a G7 chord, opposite function to the tonic. Cmaj11 is therefore a C triad and G7 chord jammed together, two opposing functions fighting in a sack .

"C11" is quite a common symbol, but should ALWAYS be assumed to mean C7sus4 or C9sus4 (the 9th being optional). It's therefore a useful shorthand for those chords, because no experienced player (IMO) is going to put a 3rd into an "11" chord.

I agree totally on "+" btw. I've got used to it being used to mean "#" - and I see an argument that #9 and #11 are technically augmented intervals (half-step more than major or perfect). But it seems like idiotic laziness to risk confusion with #5. Honestly, how much harder is it to add two more strokes to "+" to make "#" and avoid all ambiguity?
(Sometimes I think jazz musicians quite enjoy making their language impenetrable to newbies . Perish the thought... )